1. Field of the Invention
The present invention relates to a light pipe which is excellent in light utilizing efficiency and with which a liquid-crystal display device bright and easy to view can be formed, and further relates to a method for producing the light pipe.
The present application is based on Japanese Patent Application No. 2000-4240, which is incorporated herein by reference.
2. Description of the Related Art
As shown in FIG. 4, a reflection-transmission double type liquid-crystal display device comprising a side-lighting type light pipe 2 having a light source 21 on one of side surfaces of the light pipe 2, a reflection layer 3 disposed on the back surface of the light pipe 2, and a liquid-crystal display unit 4 having a liquid-crystal panel 42 and polarizers 41 and 43 disposed on the light exit side is heretofore known (see JP-A-10-106328). In the background-art reflection-transmission double type liquid-crystal display device, the light source is added while the merit of a reflection type liquid-crystal display device excellent in power saving is utilized so that the liquid-crystal device can serve as a transmission type liquid-crystal display device in a darkplace, or the like.
In a scattered reflection system through a satin-embossed structure, it was difficult to achieve bright display because exit light was shifted largely from a perpendicular direction. Therefore, the aforementioned light pipe was designed so that the optical path of light incident on one of side surfaces of the light pipe was controlled through reflection on prism-like irregular surfaces, whereby the light could be made to exit in a viewing direction with good perpendicularity to achieve display excellent in luminance. There was however a problem that display became dark in a reflection mode or in a transmission mode or a problem that an undesirable difference between darkness and brightness was formed easily in display.
It is an object of the present invention to develop a light pipe that can be used for forming a transmission type or reflection-transmission double type liquid-crystal display device which is bright, which is excellent in uniformity of brightness and which is good in display quality.
According to the present invention, there is provided a light pipe comprising: a transparent resin plate having an average in-plane retardation of not larger than 20 nm; a transparent film having an average in-plane retardation of not larger than 20 nm; and an adhesive layer having a refractive index different by 0.1 or less from that of the transparent film, wherein the transparent film is bonded to the transparent resin plate through the adhesive layer; and wherein the transparent film includes a repetitive prismatic structure constituted by optical path control slopes which are disposed on an outer surface of the transparent film and which are aligned in a substantially predetermined direction so that each of the optical path control slopes is inclined at an inclination angle in a range of from 35 to 48 degrees with respect to a film plane. There is further provided a method for producing a light pipe, comprising the steps of: forming a repetitive prismatic structure on one surface of a transparent film through a mold having a predetermined prismatic structure; providing an adhesive layer on a surface of the transparent film on which the repetitive prismatic structure is not formed; molding the transparent film and a transparent resin plate into predetermined shapes respectively; and bonding the transparent film to the transparent resin plate through the adhesive layer provided on the transparent film.
In use of the light pipe according to the present invention, there can be formed a transmission type or reflection-transmission double type liquid-crystal display device which hardly generates display unevenness caused by lowering of brightness, variation in brightness, etc., which is bright, which is excellent in uniformity of brightness and which is good in display quality. It has been found that the cause of lowering of luminance and occurrence of an undesirable difference between darkness and brightness in the background-art light pipe is a retardation of the background-art light pipe. Thus, the present invention is to reduce the retardation of the light pipe as largely as possible on the basis of this finding.
That is, the retardation of the light pipe seems to have little influence when luminance of the light pipe per se is evaluated. However, when the light pipe is used for a back-lighting system, or the like, in a liquid-crystal display device, the retardation becomes a cause of elimination or change of the polarization state. Hence, the quantity of light effectively utilized for liquid-crystal display is reduced totally or partially, so that the reduction of the quantity of light becomes a cause of lowering of luminance and occurrence of an undesirable difference between darkness and brightness. When the light pipe is used for liquid-crystal display, it is therefore necessary to reduce the retardation of the light pipe as largely as possible so that the state of polarization through polarizers, or the like, can be retained as sufficiently as possible. The background-art light pipe is generally formed by an injection molding system. In such a molding system according to the background art, a light pipe large in retardation and partially large in variation of retardation is apt to be produced because optical distortion based on molecular alignment, or the like, is apt to occur.
Hence, in the light pipe according to the preset invention, light incident on one of side surfaces of the light pipe is reflected on the optical path control slopes with good directivity so that light favorable to visibility can be generated efficiently. In such a manner, bright display can be achieved in a transmission mode. Further, when the optical path control slopes are combined with flat surfaces having an area of not smaller than 10 times as large as the area of the optical path control slopes, and when a reflection layer is provided on the back surface of the light pipe, the light pipe is excellent both in efficiency of entrance of external light and in efficiency of transmission of the light after reflection. Hence, not only bright display in a reflection mode but also bright display in a transmission mode as described above can be achieved. Hence, a transmission type or reflection-transmission double type liquid-crystal display device which is bright and which is excellent in uniformity of brightness can be obtained.